Thank you for your Question.
So the best thing a person can do for the deceased is to give charity. There is nothing like charity for the ones who have preceded us to the next world.
In your case, sincere repentance and also , if you’re able to, to reverse the sin you committed or somehow compensate it should help you and them too.
Reciting Quran for them and making speacil dua for them should give tranquility to you and them InshaAllah.
Hope it all goes well for you